Preexisting Conditions

Whether a climate event or episode exceeds the threshold to produce an ecosystem response, as well as the degree of buffering to the climate signal the ecosystem may provide, both depend on preexisting conditions in the ecosystem and, sometimes, the climate system. Certainly, we see frequent examples of preexisting conditions themselves acting as gateways, filters, and even catalysts of ecosystem response to a climate driver. Many of the studies presented in this book contain examples of the importance of preexisting conditions to later ecosystem response.

A straightforward, simple example concerned the windthrow events in winter in the northern Cascade Range of Oregon. Such events were found to occur particularly when winds were from the east after a period of dry weather. High-pressure conditions in winter gave rise to icing on the branches of trees, setting the stage for some windthrow events (chapter 19; Sinton and Jones 2002). Another straightforward example involves the white spruce in Interior Alaska, which must have a sufficient level of growth reserves as a precondition to a successful seed production event that itself would be triggered by a climate episode and the consequences thereof (chapter 12). A second example from Oregon shows that one climate event can act as a preexisting condition for a second climate event that, in turn, gives rise to an important ecosystem response. This is the case of the bark beetle outbreak following a drought that was preceded by a windthrow event. In these examples the preexisting conditions act as gateways to permit additional ecosystem response. The existence of ice on the trees might be regarded as a third element that acts as a catalyst and makes the windthrow event worse than it might otherwise have been. The other two elements present in this case are dry conditions and wind.

Other examples of the importance of preexisting conditions are less straightforward. For example, the previous disturbance and land-use history are very important in determining the exact effects of a new hurricane storm event (chapter 2). In another case, some preexisting conditions can increase the certainty that plants will suffer adverse effects, but there are also situations when the adverse effects can occur anyway if a detrimental climate event occurs. This is shown in the example of drought in the North Central Region (chapter 4). Low corn yields occurred in 1988 because of a high heat/precipitation value in July. But in this case the stage had virtually been set for low yields because of the physiological stress that had occurred in the previous May and June. On the other hand, in Michigan in 2001, May and June precipitation values were well above normal, but an exceptionally dry July and August period led to low yields for this year as well. Shallow roots for corn and soybeans were unable to make use of preexisting soil moisture at the lower level of the soil. In these agricultural examples the preexisting conditions could be said to be acting as filters to further ecosystem response.

In general, the effect of preexisting conditions is more marked at the shorter timescales, as in the previous examples. When dealing with decadal and longer timescales, the climate and related biophysical conditions become part of the preexisting conditions for the next climate episode. In the Palmer LTER example, the 60-year warming trend itself becomes part of a preexisting condition on which quasi-quintennial variation is superimposed (chapter 9). A similar pattern is seen at the Arctic LTER in Alaska, except that in this case an interannual variation is superimposed on an 11-year (so far) warming trend (chapter 5). An extreme example of how preexisting conditions do play an important role at the longer scale is at the McMurdo Dry Valleys LTER site. Here, in Taylor Valley (77.5° S), Fountain and Lyons (chapter 16) observe a strong climatic legacy whereby past climate conditions strongly imprint current ecosystem structure, function, and biodiversity. Specifically, shifting precipitation and temperature patterns caused the Ross Ice Shelf to enter Taylor Valley and impound a valley-wide lake beginning about 27,000 years ago. Ice sheet retreat, again due to changes in precipitation pattern, about 9,500 years ago caused the lake to drain. Relic benthic algal mats from the ancient lake locally increase the organic carbon content of the Taylor Valley soils. Fountain and Lyons believe the current soil communities depend on this organic carbon matter as a primary carbon source. At the longest timescale considered in this book, Monger (chapter 17) describes how climate variability between glacial and interglacial periods for the area around the Jornada LTER site can actually give rise to new geomorphic surfaces on which ecosystems develop. During times of higher precipitation in the southern New Mexico area, erosion and sedimentation markedly altered the surfaces of both river valley and piedmont areas. In addition, at this timescale, climate and vegetation actually work together to control erosion rates on piedmont slopes.

Some conceptual, as well as real biophysical, elements can be considered as preexisting conditions. We consider three examples. First, as was found in some of the ENSO cases (chapter 6), there must be a specific plant physiological linkage available for plants to respond to unusual extreme climate conditions. The rooting depth of species is a recurrent example of a physiological linkage. Such a linkage could, in some ways, be considered a preexisting condition. Second, preexisting conditions may also be thought of as a set of nonclimate-related processes that form a backdrop on which climate variability operates, or the opposite is equally true. For example, in the Konza Prairie such factors or processes include fire, nutrients, grazing by large ungulates, soil characteristics, and topography (chapter 20). Another example relates to the timing of preexisting conditions. For some tree species at Coweeta, the degree of effect of a drought is in part determined by the stage of the cycle of the Southern Pine Beetle population (chapter 3). Third, we should also consider the issue of preexisting conditions in terms of the fact that most of our ecosystems are chaotic systems. It is an inherent characteristic of chaotic systems that a small change in initial conditions may lead to large changes in subsequent conditions. With respect to climate warming, Shaver et al. (2000, p. 880) point out that "the same temperature change applied to different ecosystems will illicit different responses depending on initial position on the temperature response surface . . . and on initial biogeochemical conditions and composition. . . ." Analogous situations are found throughout the natural world and in the examples given in this volume.

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